Great White Spot

The Great White Spot, also known as Great White Oval, on Saturn, named by analogy from Jupiter's Great Red Spot, is a name given to periodic storms that are large enough to be visible by telescope from Earth by their characteristic white appearance. The spots can be several thousands of kilometers wide.

Occurrence

The phenomenon is somewhat periodic at 28.5-year intervals, when Saturn's northern hemisphere tilts most toward the Sun. The following is a list of recorded sightings; years with spots generally considered to be part of the cycle are 1876, 1903, 1933, 1960, and 1990.

* 1876 - Observed by Asaph Hall. He used the white spots to determine the planet's period of rotation.
* 1903 - Observed by Edward Barnard.
* 1933 - Observed by Will Hay, comic actor and amateur astronomer. Until recent times the most celebrated.
* 1960 - Observed by JH Botham (South Africa).
* 1990 - Observed by Stuart Wilber, from 24 September through November.
*1994 - Studied by ground-based observers and the Hubble Space Telescope. [ [http://hubblesite.org/newscenter/archive/releases/1994/53/text/ HubbleSite - NewsCenter - Hubble Observes A New Saturn Storm (12/21/1994) - Release Text ] ]
* 2006 - Observed by Erick Bondoux and Jean-Luc Dauvergne.

That none were recorded before 1876 is a mystery, in some ways akin to the long observational gap of the Great Red Spot in the 18th and early 19th centuries; the 1876 GWS was extremely prominent, being visible in apertures as small as 60 mm. Was the earlier record simply poor, or was the 1876 GWS truly a first for the telescopic era? Some believe that neither scenario is likely. [Patrick Moore, ed., "The 1993 Yearbook of Astronomy", Mark Kidger, "The 1990 Great White Spot of Saturn", (London: W.W. Norton & Company, 1992), 179]

Mark Kidger (1992) has described three significant GWS patterns:
#The GWSs alternate in latitude, with one apparition being limited to the North Temperate Zone (NTZ) or higher, and the following being limited to the Equatorial Zone (EZ). For instance, the 1960 GWS was high-latitude, and the 1990 GWS was equatorial.
#The high-latitude GWSs recur at a slightly shorter interval than the equatorial GWSs (~27 versus ~30 years).
#The high-latitude GWSs tend to be much less prominent than their equatorial counterparts.

Based on these apparent regularities, Kidger forecast that the next GWS will occur in the NTZ in 2016, and will probably be less spectacular than the 1990 GWS. [Ibid., 180.]

Characteristics and causes

A "classic" Great White Spot is a spectacular event, in which a brilliant white storm enlivens Saturn's usually-bland atmosphere; all the major ones have occurred in the planet's northern hemisphere. [Ibid., 178.] They typically begin as discrete, literal "spots", but then rapidly expand in longitude, as the 1933 and 1990 GWSs did; in fact, the latter eventually lengthened enough to encircle the planet. [Ibid., 187-9.] Current theory suggests that GWSs are massive atmospheric upwellings, perhaps due to thermal instability. [Ibid., 211-12.]

The rough coincidence of the GWSs with the summer solstice in Saturn's atmosphere has been seized on as proof that insolation is the main cause, though their occurrence has been more closely linked with time elapsed since the northern "winter" solstice. Whether similar phenomena occur during the winter solstice as well is unknown, as the rings effectively "block" a terrestrial view of the hemisphere. [Ibid., 213-4.]

Notes

References

* [http://hubblesite.org/newscenter/newsdesk/archive/releases/1991/04/ 1990/1] Hubble Space Telescope image
* [http://antwrp.gsfc.nasa.gov/apod/ap060127.html 2006:] observed with a 12" telescope by amateurs near Paris.
* [http://www.nasa.gov/vision/universe/solarsystem/Saturn_storm.html Volunteers Help NASA Track Return of the Dragon]
* Patrick Moore, ed., The 1993 Yearbook of Astronomy, Mark Kidger, "The 1990 Great White Spot of Saturn", 176-215, (New York: W.W. Norton & Company, 1992).
*

External links

* [http://www.spacetelescope.org/bin/images.pl?searchtype=freesearch&string=Great+White+Spot The Great White Spot at ESA/Hubble]